Method for detecting wires using the ROSAR system

ABSTRACT

A ROSAR wire detection method is based upon ROSAR focusing of entire segments of wire. By generating a wire reference signal comprised of a sum of coherent reference signals, the basis for reliable wire detectability is provided.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This application claims the priority of German patent document101 01 990.4, filed Jan. 18, 2001, the disclosure of which is expresslyincorporated by reference herein.

[0002] The invention relates to a method for detecting wires using asynthetic aperture radar system based upon rotating antennas (ROSAR).

[0003] One of the greatest dangers in helicopter flight is that thepilot might overlook a wire obstacle located across his flight path,which could result in a serious accident or crash. This risk isincreased under adverse weather conditions, such as rain, fog, snowfall,etc. The systems currently provided on board a helicopter are purelypassive systems, such as residual light amplifiers, heat imagingsystems, etc., which cannot be used under conditions of fog, smoke, andprecipitation, due to their limited range. And recently implementedoptical, electro-optical, and laser-based imaging systems also cannotsignificantly improve the situation in terms of detecting wire obstaclesunder adverse weather conditions, fog, or smoke.

[0004] German patent documents DE 39 22 086 and DE 43 23 511 disclose aROSAR system that operates on-line in near real time, and can be usednot only in mapping and mine detection applications, but also fordetecting obstacles. The provided detection of wires via reflectionpoints in various resolution cells, however, assumes a strong dominanceof the reflection of the wire cross sections within a resolution cell.If this prerequisite is not met, costly postprocessing of the ROSARimages may improve the detectability of wires; however this method isnot only costly—as mentioned above—it also cannot be consideredsufficiently accurate.

[0005] One object of the present invention is to provide a method whichguarantees optimized wire detectability based upon a ROSAR focusingsystem.

[0006] This and other objects and advantages are achieved by thedetection method according to the invention, which is based on ROSARfocusing of entire segments of wire. By generating a wire referencesignal comprised of a sum of coherent reference signals, reliable wiredetectability is provided.

[0007] As shown in FIG. 5, ROSAR rings are generated by a rotatingreceiving antenna located on the tip of a rotor blade. The rotatingreceiving antenna itself describes a circle with a radius being thelength of the rotor blade (not illustrated). The wire, which is disposedat a distance Ro from the position of the helicopter, extends overseveral adjacent range rings. When a helicopter approaches a wireobstacle, the approach of the wire obstacle to that circle isnecessarily always tangential, independent of the position of thehelicopter and the position of wire obstacle. The continued approach isthen always parallel to this. Thus, the number of possibilities forapproaching is reduced. For further calculation it is assumed that thewire obstacle is a straight wire segment. This assumption issufficiently accurate because a wire drawn between two sustainers isapproximately straight. Consequently, for determining the wire referencesignal a straight wire segment is assumed.

[0008] Other objects, advantages and novel features of the presentinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic illustration of various positions for asegment of wire within a ROSAR range ring, as this ring is approached;

[0010]FIG. 2 is a schematic diagram which illustrates the wire detectionprocess;

[0011]FIG. 3 is a schematic diagram of the calculation of the wirereference signal;

[0012]FIG. 4 shows ROSAR rings which are generated by the rotatingreceiving antenna located on the tip of a rotor blade; and

[0013]FIG. 5 shows the configuration of a ROSAR system for detectingwires according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] ROSAR (Rotating Synthetic Aperture Radar)—in other wordssynthetic aperture radar based upon rotating antennas—the potential fordetecting a wire obstacle under adverse weather conditions, as hasalready been demonstrated experimentally. The problem exists, however,that the wavelengths that are used must lie within the magnitude of thediameter of the wire obstacle to be detected; hence the radar reflectioncross section is relatively small.

[0015] In the simulation of high-tension wires or lines, modeling of thewire obstacle is provided by a smooth rod, which, due to its reflectiveproperties, offers reflection signals only in the case of a verticalincidence of the transmitting signal. In practice, however, high-tensionlines that pose a danger to helicopters are comprised of individualcoiled wires, creating reflection points that lie close to one anotherbut have a small reflection cross section. The actual problem is that itis not always certain that the radar reflection cross section of thehigh-tension cable in a resolution cell will be greater than thereflection cross-section of a wire-free resolution cell.

[0016] In traditional SAR or ROSAR signal processing methods, thereceived signal mix is cross-correlated with a reference signal, whichis the response of a punctiform reflector to irradiation with thetransmitted signal from the moving transmitting and receiving antenna.This causes the received signal energy of a reflection point to befocused over the entire rotating angle of the rotor, but only thatsignal energy that originates from a single reflection point.

[0017] According to the present invention, a special “wire referencesignal” is generated, which is then used to focus all partial signalsthat originate from a section of wire. This wire reference signal isnaturally dependent upon the position of the wire. Hence, a wide varietyof wire reference signals is created, all of which must be correlatedwith the received signal, which would ordinarily require management of adubiously large number of calculations. The variety of wire referencesignals, however, can be limited substantially by means of a specialtechnique which uses a straight wire that a helicopter is approachingand that enters a ROSAR range ring (FIGS. 1 and 4). As noted previously,the approach to a range ring in the ROSAR system is always tangential,due to the system's panoramic (360°) field of view. The continuedapproach is then parallel to this.

[0018] This is illustrated in FIG. 4, which shows the basic concept ofthe invention. As noted, when a helicopter approaches a wire obstacle awire segment of the straight wire obstacle is tangential to the circle,which is described by the rotating rotor blade. For this wire segment,which extends over several adjacent range rings, a wire reference signalis determined, as the coherent sum of partial signals which arereflected along the segment of wire. For determining the wire referencesignal only, the partial signals of the wire segment−α_(max)≦α_(n)≦α_(max) are taken into account. The wire reference signalis stored in a wire reference signal memory whereby the storagepositions are addressed by the transit time τ_(n) and the distanceR_(n). (See FIG. 5.)

[0019] A correlator is used to correlate the wire reference signal fromthe wire reference signal memory with the current receiving signal froma receiving signal memory, as shown in FIG. 5. The correlation signalcan be displayed, and can also be analyzed so that if it exceeds a giventhreshold the wire alarm is set off (FIG. 5.)

[0020] As is known from traditional ROSAR signal processing methods, atthe angle position α_(R) the transmitting antenna transmits pulsesignals, and the reflected signals of a certain transit time are storedin a respective range ring memory for that transit time. According tothe invention the reflected partial signals which are characterized bythe rotor position α_(R) and the transit time$\tau_{n} = \frac{2\quad R_{0}}{{c \cdot \cos}\quad \alpha_{n}}$

[0021] are stored in the current receiving signal memory. This is afundamental difference between the invention and the traditional ROSARsignal processing methods.

[0022] In contrast to known ROSAR processing systems, cross-correlationof the received signal according to the method of the invention is notperformed using the point reference signal along the angle of rotationin a single range ring; rather the entire received signal from a sectorin which the wire obstacle is supposedly located is correlated with thewire reference signal (FIG. 2).

[0023] In the case of a ROSAR embodiment comprising a transmittingantenna that is fixed on the fuselage and a receiving antenna that ismounted near the tips of the blades, the wire detection processaccording to the invention includes the following steps: In the rotorposition −α_(R,max) a pulse is transmitted, and the backscatteredreceived signals are stored in the memory position −α_(max) (FIG. 2). Inthe meantime, the rotor has turned farther to the angle α_(R), and thesame process is repeated, until the rotor has reached +α_(max). Thisreceived signal which is dependent upon the angle of rotation α of therotor blade (and hence, of the receiving antenna) is correlated with thewire reference signal, which is also dependent upon the angle ofrotation.

[0024] It should be noted that the angle range between −αmax and+α_(max) is the full width of half maximum of the antenna receivingdiagram. The angle range can vary between a small angle and 180 degreesand is defined by the design of the antenna aperture. α_(S) indicatesthe angle under which a partial signal of the backscattered (reflected)signal of the wire segment is received.

[0025] To generate the wire reference signal in accordance with theinvention, the received signals for the individual reflection points ofthe wire are calculated, taking into account the different transittimes, these reflection points lying along a single line, which isshifted geometrically. Afterward, the partial received signals arecalculated based upon the rotating antennas, also taking intoconsideration the different transit times and doppler shifts, and arecombined to form a single “wire reference signal”, as is illustrated inFIG. 3. In the presence of irregular circular motions of thetransmitting and receiving antennas, an optimization via application ofthe Rechenberg Optimization Method for ROSAR (DE 198 51 910 C1) may alsobe implemented.

[0026] Expediently, discrete reflection points, which are characterizedby their distance from the vertical axis (α_(R)=0 in FIG. 3), areassumed on the segment of wire to be detected. For this purpose, it isassumed, that the reflection of the wire is diffuse along its entirelength. (The reflection points of the wire are not predetermined but areestablished just for modeling.) Because of the assumption that thereflection of the wire is diffuse, a partial reflection signal of thewire can be seen as having its origin at an adjacent point on thesegment of wire. The total reflection signal of the segment of wire isthus comprised of the sum of all partial reflection signals of the wire.

[0027] The reference signal S_(Ds) for the segment of wire is thuscomprised of a sum of partial signals, as follows:$S_{Ds} = {\sum\limits_{\alpha_{R} = {- \alpha_{R\quad \max}}}^{\alpha_{R} = \alpha_{R\quad \max}}{\sum\limits_{v = 1}^{n}{S_{v}2\quad {\pi \left( {f_{T} + {\frac{2V}{c}f_{T}\quad \sin \quad {\alpha_{v}\left( X_{v} \right)}}} \right)}t}}}$

[0028] Herein, ƒ_(T) refers to the carrier frequency, in other words thetransmitter frequency, V is the tangential speed of the rotor, and c isthe speed of light.

[0029] The distance between adjacent reflection points is arbitrary, solong as the phase information of coherent waves is maintained. Thisrequirement is fulfilled with a distance which is less than {fraction(1/16)} of the transmitter wavelength. (The reflection points, it shouldbe noted, are in fact a kind of abstract points, which are defined onlyfor calculation of the wire reference signal.) Thus, referring to FIG.3, for each rotor position α_(R) the sum of the receiving signals of alldiscrete reflection points of the segment of wire is performed. Thereflection points are characterized by their distance X_(v) from thevertical axis (α_(R)=0). The parameter v runs from 1 to an upper limitn, with the requirement that the distance between adjacent points isless than {fraction (1/16)} of the transmitter wavelength. Thiscalculation is performed for each rotor position α_(R) between −α_(max)and α_(max) to achieve the total reference signal for the wire segment.

[0030] A fundamental difference between a point reference signal and thewire reference signal described herein is that in the case of the pointreference signal, for each angle of rotation α_(R) of the antennaposition, only one received signal with only one discrete doppler shiftoccurs, while with the wire reference signal a sum of signals ofdifferent frequencies occurs.

[0031] The foregoing disclosure has been set forth merely to illustratethe invention and is not intended to be limiting. Since modifications ofthe disclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A method of operating a ROSAR synthetic apertureradar system based upon rotating antennas that operates at pulsefrequency or in FM-CW operation to detect wire obstacles on the basis ofa ROSAR focusing of entire segments of wire, said method comprising:establishing wire reference signals from the sum of coherent referencesignals whose points of origin lie along a straight segment of wire;coherently processing said wire reference signals; and correlating anentire received signal from one segment with the wire reference signal.2. The method in accordance with claim 1, wherein: received signals thatare dependent upon an angle of rotation α of a rotating antenna arecorrelated with the wire reference signals; and the wire referencesignals are dependent upon the angle of rotation.